Patent Application
20190326401
Aspects of the present disclosure relate to silicon-on-insulator devices, and more particularly, to structures and methods for connecting body of a silicon-on-insulator MOSFET.
Silicon-on-insulator (SOI) technology refers to the use of a layered silicon-insulator-silicon substrate in place of a conventional silicon substrate in semiconductor manufacturing, especially microelectronics, to reduce parasitic device capacitance, thereby improving performance. An integrated circuit built using SOI devices may show processing speed that is 30% faster than a comparable bulk-based integrated circuit and power consumption being reduced by as much as 80%, which makes it ideal for mobile devices. SOI chips also reduce the soft error rate, which is data corruption caused by cosmic rays and natural radioactive background signals. SOI transistors offer a unique opportunity for CMOS architectures to be more scalable. The buried oxide layer limits the punch-through that may exist on deep sub-micron bulk devices.
Due to the existence of the buried oxide layer, the body of an SOI MOSFET is often floating in circuit design, meaning no connection of the body to a bias voltage. Floating body of an SOI MOSFET results in an effect called floating body effect, a dependency of the body potential on the history of the SOI MOSFET's biasing and the carrier recombination processes. For many applications, leaving body floating causes undesired effects such as kinks in the output characteristics, leading to non-linearity, reduced breakdown voltage, and degraded reliability. For such application, body connection may be needed. However, conventional body connection approaches often comes at a cost of reduced device performance and/or increased device size. Accordingly, it would be beneficial to provide a body connection scheme without substantial performance or area penalty.
The following presents a simplified summary of one or more implementations to provide a basic understanding of such implementations. This summary is not an extensive overview of all contemplated implementations, and is intended to neither identify key nor critical elements of all implementations nor delineate the scope of any or all implementations. The sole purpose of the summary is to present concepts relate to one or more implementations in a simplified form as a prelude to a more detailed description that is presented later.
In one aspect, a silicon-on-insulator device comprises a back insulating layer and a semiconductor layer on the back insulating layer. The semiconductor layer includes a source region of a first conductive type having a front source surface and a back source surface, a channel region of a second conductive type having a front channel surface and a back channel surface, and a drain region of the first conductive type. The silicon-on-insulator device further comprises a gate insulating layer on the front channel surface of the channel region and a back silicidation layer on at least a portion of the back source surface and a portion of back channel surface.
In another aspect, a method comprises providing a silicon-on-insulator wafer having front metal connection system, a MOSFET, a back oxide layer, and a sacrificial substrate. The MOSFET includes a source region having a front source surface and a back source surface, a drain region, and a channel region having a front channel surface and a back channel surface. The method further comprises bonding the silicon-on-insulator wafer to a handle wafer, removing the sacrificial substrate, patterning and etching the back insulating layer to expose a portion of the back source surface and the back channel surface of the MOSFET, and forming a back silicidation layer on the source region and the channel region through the exposed back source surface and back channel surface.
To accomplish the foregoing and related ends, one or more implementations include the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative aspects of the one or more implementations. These aspects are indicative, however, of but a few of the various ways in which the principles of various implementations may be employed and the described implementations are intended to include all such aspects and their equivalents.
The detailed description set forth below, in connection with the appended drawings, is intended as a description of various aspects and is not intended to represent the only aspects in which the concepts described herein may be practiced. The detailed description includes specific details for the purpose of providing an understanding of the various concepts. However, it will be apparent to those skilled in the art that these concepts may be practiced without these specific details. In some instances, well-known structures and components are shown in block diagram form in order to avoid obscuring such concepts.
Semiconductor-on-insulator (SOI) devices are widely used for their excellent electrical properties including lower threshold voltage, smaller parasitic capacitance, less current leakage and good switching property, etc. Conventionally, the silicon film in the channel region (body) is electrically floating. Leaving the body floating complicates device behavior due to floating body effect, such as parasitic bipolar effect, kink effect, history-dependent characteristics, etc. The floating body effect causes use of SOI devices in certain applications, such as RF, mixed signal, or high speed circuit design, challenging. The traditional remedy to the floating body effect is to provide body connection. However, the efficiency of the conventional body connection is limited and such connection often degrades device performance and consumes significant device area.
In many circuit designs, the body of an NMOS transistor is connected to a ground while the body of a PMOS transistor is connected to a supply voltage. For an NMOS transistor whose source is connected to the ground or a PMOS transistor whose source is connected to the supply voltage, the body and the source of the transistor are electrically coupled.
The MOSFET 300 further comprises a back silicidation layer 318 on at least a portion of the back source surface 314b of the source region 314 and a portion of back channel surface 304b of the channel region 304. The back silicidation layer 318 electrically couples the channel region 304 to the source region 314. Thus, through source region 314, the channel region 304 may be connected to a supply voltage for a PMOS transistor or a ground for an NMOS transistor. A separate body connection or body contact is not needed.
The back silicidation layer 318 is formed through a silicidation process, an anneal process resulting in the formation of metal-silicon alloy (silicide) to act as a contact or contact interface for low contact resistance. For example, Titanium may be deposited on silicon to form TiSi2 as a result of silicidation. Other suitable materials are possible, such as CoSi2, NiSi, etc.
The MOSFET 300 may further comprise a front silicidation layer 316 on the front source surface 314f and a front silicidation layer 326 on the drain region 324. The front silicidation layer 316 provides an interface for connection of the source region 314, and thus the channel region 304, to a front metal connection system 342. The front metal connection system 342 may include contacts, vias, and multi-level metal layers. The front metal connection system 342 may connect the source region 314 to a supply voltage for a PMOS transistor or a ground for an NMOS transistor. The front metal connection system 342 may connect the source region 314 to other signals.
The MOSFET 300 may also comprise a back metal connection system 332. The back metal connection system 332 may include contacts to the back silicidation layer 318 and may also include vias and one or more other metal layers. The source region 314 and/or the channel region 304 may be connected to a supply voltage or a ground or a signal through the back metal connection system 332.
The MOSFET 300 may further comprise a spacer 310. The spacer 310 electrically isolates the source region 314 and the front silicidation layer 316 from the gate conducting layer 308.
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At 504, the SOI wafer is bonded to a handle wafer (e.g., the handle wafer 444). After the bonding of the handle wafer, the sacrificial substrate is removed, exposing the back insulating layer.
At 506, the back insulating layer is patterned and etched with an opening (e.g., the opening 446). The opening exposes a portion or all of the back source surface and a portion or all of the back channel surface.
At 508, a back silicidation layer (e.g., the back silicidation layer 318 or 418) is formed in the exposed back source surface and the exposed back channel surface. The back silicidation layer electrically couples the channel region to the source region. Thus, through source region, the channel region may be connected to a supply voltage for a PMOS transistor or a ground for an NMOS transistor.
At 510, a back metal connection system (e.g., the back metal connection system 332 or 432) may be formed. The back metal connection system may include contacts to the back silicidation layer, vias, and one or more metal layers. The source region and/or the channel region may be connected to a supply voltage or a ground or a signal through the back metal connection system.
The previous description of the disclosure is provided to enable any person skilled in the art to make or use the disclosure. Various modifications to the disclosure will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other variations without departing from the spirit or scope of the disclosure. Thus, the disclosure is not intended to be limited to the examples described herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
